This invention relates to a metal-oxide-semiconductor (MOS) device, and in particular an improved MOS device wherein a polycrystalline semiconductor region is buried in a monocrystalline semiconductor substrate at an isolation region between elements of the device.
In conventional MOS devices, the isolation regions between elements are generally formed by selective oxidation of the substrate. However, such conventional devices formed by selective oxidation of the substrate exhibit disadvantages which prohibit further miniaturization of the devices. Specifically, an uneveness of the surface of the semiconductor device occurs which results in crystal defects of the devices.
Generally, in accordance with the prior art, after field ion implanation, an oxide film layer is selectively formed by thermal oxidation utilizing a silicon nitride mask in order to isolate the MOS elements. These devices in accordance with the prior art exhibit the following disadvantages.
1. Miniaturization of the semiconductor device is limited since the oxide film tends to migrate in the lateral direction during thermal oxidation. This oxide film then undercuts the silicon nitride film and the dimensional accuracy of the pattern is not maintained. This phenomenon is called the "bird's beak problem".
2. Further miniaturization of the semiconductor device is limited since the implanted field ions tend to migrate in a lateral direction during thermal oxidation.
3. Surface smoothness is reduced since the oxidized portion tends to thicken on the thermally oxidized semiconductor substrate.
4. The electrical characteristics deterioate since the cyrstal defects occur at the substrate at the end of the silicon nitride film at the time of thermal oxidation. Accordingly, it would be desirable to provide an MOS device which did not suffer from these defects.